In recent optical fiber transmission, a multilevel modulated optical signal encoded in two bits or more per one symbol has been applied in place of a conventional binary modulated optical signal in order to cope with an increase in channel capacity. In an optical transmission system utilizing the multilevel modulated optical signal, there has been used a multilevel optical transmitter that generates a multilevel modulated optical signal from an input optical signal and an input electrical data sequence.    Patent Document 1: Japanese Patent No. 5729303    Patent Document 2: Japanese Patent No. 5254984    Non-Patent Document 1: X. Wu et al., “A 20 Gb/s NRZ/PAM-4 1V transmitter in 40 nm CMOS driving a Si-photonic modulator in 0.13 μm CMOS”, IEEE International Solid-State Circuits Conference (ISSCC) 2013, 7.7    Non-Patent Document 2: Poulin et al., “107 Gb/s PAM-4 Transmission over 10 km Using a SiP Series Push-Pull Modulator at 1310 nm”, European Conference on Optical Communication (ECOC) 2014, Mo. 4.5.3    Non-Patent Document 3: T. Baba et al., “25-Gb/s broadband silicon modulator with 0.31-V·cm VπL based on forward-biased PIN diodes embedded with passive equalizer” Optics Express, vol. 23, pp. 32950
The multilevel optical transmitter to be used for optical fiber transmission has a function of encoding transmission data into a multilevel signal and modulating the intensity and the phase of an optical signal by this multilevel signal to output the resultant optical signal applied thereto. There are some configuration examples of an optical modulator and a driving unit that perform the multilevel optical modulation. One of the examples is the combination of parallelized binary output drivers, which are equal to the number of bits of a multilevel signal, and a Mach-Zehnder (MZ) optical modulator into which segmented phase shifters are loaded (see Non-Patent Document 1, for example). In this case, waveform deterioration caused by nonlinearity of the optical modulator and the driving unit hardly occurs, and a low-power CMOS inverter can be applied to drivers of the driving unit. Therefore, the above-described example is a configuration effective for fabricating a high-performance and low-power optical transmitter.
FIG. 1 is a schematic diagram illustrating a typical configuration example of a conventional optical transmitter. FIG. 1 illustrates an optical transmitter of four-level pulse amplitude modulation (Pulse-Amplitude-Modulation 4: PAM4) that is the simplest among multilevel optical signals.
This optical transmitter is configured by including a driving circuit 101 and a MZ optical modulator 102. The driving circuit 101 includes a MSB driver 111 and a LSB driver 112 that are CMOS inverters to amplify binary signals of the MSB and the LSB being configuration bits of an input electrical data sequence, and output the amplified binary signals. The MZ optical modulator 102 has one end thereof set as an input portion of a DC light and has the other end thereof set as an output portion of a modulated light, and includes arms 115, 116 that are a pair of optical waveguides to separate from each other between a pair of optical couplers 113 and 114.
A first phase shifter 117a is loaded in the arm 115, and a second phase shifter 117b is loaded in the arm 116. The first phase shifter 117a is segmented into a first segmented phase shifter 117a1 and a second segmented phase shifter 117a2 so as to have a length thereof segmented at a ratio of 2:1 between the MSB side and the LSB side. The second phase shifter 117b is segmented into a first segmented phase shifter 117b1 and a second segmented phase shifter 117b2 so as to have a length thereof segmented at a ratio of 2:1 between the MSB side and the LSB side. Concretely, the first segmented phase shifters 117a1, 117b1 each have a length 2L, and the second segmented phase shifters 117a2, 117b2 each have a length L.
In this optical transmitter, an electrical signal, which is transmission data, is input to the MSB driver 111 and the LSB driver 112 in the driving unit 101 as a binary signal every MSB and LSB being the configuration bits of the input electrical data sequence. The MSB driver 111 and the LSB driver 112 each are a differential amplifier, and one of differential signals amplified in the MSB driver 111 is input to the first segmented phase shifter 117a1 and the other of the amplified differential signals is input to the first segmented phase shifter 117b1. One of differential signals amplified in the LSB driver 112 is input to the second segmented phase shifter 117a2 and the other of the amplified differential signals is input to the second segmented phase shifter 117b2. The first phase shifter 117a and the second phase shifter 117b each modulate an optical phase according to the differential signals supplied from the MSB driver 111 and the LSB driver 112.
Here, signal amplitudes of the electrical signals to be output from the MSB driver 111 and the LSB driver 112 are the same as one another, and the first segmented phase shifter 117a1 (117b1) and the second segmented phase shifter 117a2 (117b2) are the same as each other in an optical phase shift amount per unit length. Each phase shifter length of the first segmented phase shifter 117a1 (117b1) and the second segmented phase shifter 117a2 (117b2) is weighted. Therefore, the shift amount of the optical phase to be changed by the signal of the MSB and the shift amount of the optical phase to be changed by the signal of the LSB result in 2:1, and the following is obtained as the entire optical phase change ϕ(t).ϕ(t)=21·MSB(t)+20·LSB(t)With ϕ(t), a modulated signal results in four values of 0, 1, 2, and 3. The MZ optical modulator 102 has a function of changing each phase change in the arms 115, 116 into an optical intensity change. Therefore, an output optical signal to be obtained consequently results in a four-level intensity-modulated signal (PAM4) responsive to ϕ(t).
The above-described optical transmitter needs to have a sufficiently long separated region between the first segmented phase shifter 117a1 (117b1) and the second segmented phase shifter 117a2 (117b2) in order to prevent mutual mixing of the respective electrical signals (see Non-Patent Document 2, for example).
Further, between the first segmented phase shifter 117a1 (117b1) and the second segmented phase shifter 117a2 (117b2), a phase shift associated with propagation of optical signals is caused. There is considered a case where signal timings are regulated at respective middle portions between the first segmented phase shifter 117a1 (117b1) and the second segmented phase shifter 117a2 (117b2) as illustrated in FIG. 1. In this case, the following is obtained as an optical signal delay τ between the first segmented phase shifter 117a1 (117b1) and the second segmented phase shifter 117a2 (117b2).
τ≈(1.5 L+Ls)·ng/c (L: first phase shifter length, Ls: separated region length, ng: optical mode group refractive index, c: light velocity)
In the case of L=500 μm, Ls=25 μm, and ng=4.0, τ=10 p seconds is satisfied.
FIG. 2 is a characteristic chart illustrating output waveforms of 25 Gbaud PAM4 obtained when the optical transmitter is driven with the MSB and the LSB set the same in timing without compensation for a signal delay.
As illustrated in FIG. 2, the rising timings at level transitions of 0, 1, 2, and 3 are each shifted by a signal delay between MSB transition and LSB transition. Due to this effect, an effective eye opening width common among three eye patterns in the signal waveform of PAM4 is narrowed to cause a problem when performing determination⋅decoding on the optical reception side. In order to solve such a problem, it is necessary to give a delay corresponding to the optical signal delay between the MSB driver and the LSB driver on the driving unit side. However, in order to constantly give a necessary delay amount while suppressing the effect of manufacturing variation, a delay amount varying mechanism, a monitor mechanism, and so on are needed, resulting in that problems of an increase in scale of the transmitter and an increase in power consumption resulting from this increase are caused.